The present invention relates generally to an apparatus for preventing cave-ins of mining galleries and underground excavations, and in particular to an improvement of such an apparatus.
There are many instances where the roof of an underground excavation must be prevented from caving in; by way of example, the prior art and the present invention will be discussed hereafter with reference to the prevention of cave-ins in underground mining galleries.
Equipment already exists for preventing such cave-ins, so called mine roof supports having a base supported on the floor of the excavation, an upstanding shield which is connected with the base and rises upwardly from the same, and a pit prop of fixed or telescopic type by means of which the shield can be raised or lowered with reference to the base so as to accommodate the apparatus to excavations of different height. An example of this general type of apparatus can be found in U.S. Pat. No. 3,672,174, and another example can be found in U.S. Pat. No. 3,691,775. This type of equipment is provided with advancing means to make it possible for the entire apparatus to advance towards and away from the mine face, that is the face of the excavation where work is being performed, for example to remove coal or the like. Usually, such apparatus is used in conjunction with mining machinery which removes the coal or other material from the mine face, so that the coal can fall onto scraper conveyors or the like to be transported along the mine face and away from the operating station.
One type of mine roof support equipment uses a carriage located at the supporting base and being connected with a cylinder and piston unit which is also secured to the base itself, for instance which has its cylinder secured to the base and its piston rod secured to the carriage. The latter extends in direction substantially normal to the mine face and has a free end that can be releasably connected with the scraper conveyor or similar element, either directly or indirectly. This free end is located remote from the opposite end of the carriage which is connected with the piston rod, and the scraper conveyor or the like is located intermediate the end face and the base of the mine roof supporting apparatus. This means that when the cylinder and piston unit is operated to retract its piston and piston rod, the entire unit is shifted towards the mine face, due to the fact that the free end of the carriage is firmly held by the appropriately anchored conveyor.
In underground excavations, especially in mining galleries and the like, the height of the excavation varies frequently as the removal of material from the mining face progresses, for instance because a coal seam is of different height at different locations. It is important from the point of view of safety that the roof supporting apparatus be capable of being accommodated to these height variations as soon as they occur, for instance as soon as material is removed to a greater height at one spot than at a previous spot. In this respect however, problems have been encountered with the type of equipment known in the prior art, and particularly with a type of mine roof support which is constructed in the manner discussed earlier but wherein the roof supporting shield is provided, at its free edge that is located closest to the mine face, with a roof supporting cap that is pivoted to this free edge and bridges the spacing between the free edge and the mine face, which spacing corresponds approximately to the spacing between that edge of the base that is closest to the mine face, and the mine face itself. When the height of an excavation changes, for instance increases, and an apparatus of this type is adjusted by raising the shield and therefore the roof supporting cap, a difficulty is experienced which results from the manner in which this type of apparatus is constructed, namely that the edge of the roof supporting cap that is located closest to the mine face, and that should advantageously be located right at the mine face to leave no gap through which a break-in might occur, will immediately tilt downwardly away from the roof of the excavation. This means that the more the shield and the cap are raised, the more the free edge of the cap will move away from the mine face, thus forming with the same a gap which increases the more, the higher the shield and the roof supporting cap are raised. This brings with it the evident danger that in these regions where there is now no longer any support for the roof, the material of the roof might cave in.
The prior art has proposed to overcome this problem by providing the roof supporting cap itself with extensions that can be moved out of the cap towards the mine face in order to bridge the gap which develops. However, it has been found that this type of construction presents very significant difficulties in structural and technological respects. Such extensions are subjected to extremely high stresses since the upwardly directed supporting force of the pit prop or analogous device used for holding up the shield and the roof supporting cap against the downward action of the weight of the roof material, must be transmitted into these extensions and via the same to the roof material. This meant extremely complicated and expensive structural measures had to be taken, which greatly increased the expense of this type of apparatus.